Microwave oven having a mode stirrer located within the waveguide



April 15, 1969 R. COUGOULE 3,439,143 MICROWAVE OVEN HAVING A MODE STIRRER LOCATED WITHIN THE WAVEGUIDE Filed Dec. 8, 1966 x 5 t 7 j JNVENTOR. ,eader/ United States Patent Office 3,439,143 Patented Apr. 15, 1969 3 439 143 MICROWAVE OVEN IIAEING A MODE STIRRER LOCATED WITHIN THE WAVEGUIDE Robert L. Cougoule, San Carlos, 'Calif., assignor to Litton Precision Products, Inc., Palo Alto, Calif., a corporation of Delaware Filed Dec. 8, 1966, Ser. No. 600,082 Int. Cl. H05b 9/06 U.S. Cl. 219-1055 12 Claims ABSTRACT OF THE DISCLOSURE A microwave oven includes a source of microwave energy, an oven cavity and a waveguide which connects the source to the cavity. The waveguide has a short circuited end remote from the source end and includes at least one microwave passage for feeding microwave energy into the oven cavity. This first microwave passage is spaced from the short circuited waveguide end. Located within the waveguide and between the passage and short circuited end is an elongated strip of conductive material. This elongated strip reflects microwave energy. An axis of rotation is provided and the reflective strip is supported, spaced from and substantially non-coplanar with that axis of rotation or off-axis."

The present invention relates to a microwave oven for heating foods, and more particularly, to an improved microwave oven which provides uniform heating patterns within the oven cavity by means of a moving or rotating mode stirrer located within the waveguide, which couples microwave energy between the source and the cavity.

In known microwave cooking or heating devices, standing waves of high frequency are set up within the oven cavity due, for example, to reflection of the supplied high frequency waves or microwaves, as variously termed, from the cavity walls. Such standing waves include locations of high electric field intensities and low electric field intensities at different positions within the oven cavity. This distribution results in relatively hot or cold spots at various locations therein, and with a large piece of food or with several pieces placed about the cavity, causes some portions of food to be heated more than other portions. This result is obviously undesirable.

In order to avoid these hot and cold spots, it has generally been the practice to increase the number of modes excited within the cavity by the addition of either fixed or moving elements. This is possible since a cavity having its length, width, and height different in dimension, each large relative to the wave length of the microwave source, is inherently capable of sustaining a plurality of different modes. By adding together the electric fields of many different modes, a substantially more uniform electric field is obtained than is obtainable with a single dominant mode and the magnitude of the differences between the high and low electric field intensities at different locations within the cavity is considerably reduced.

The usual solution for this problem is the mode stirrer, a fan-like unit consisting of many blades located within the cavity, each of which reflects incident microwave energy within the cavity. By rotating the stirrer within the cavity, the aforesaid high and low field intensity spots, or hot and cold spots, are sequentially shifted to different locations around the oven and each portion of a large piece of food on the average over a period of time receives approximately the same amount of heating energy as it is sequentially exposed to high and low intensity fields.

Although this method of obtaining a more desirable heating field within the cavity has proven satisfactory, it

possesses the obvious disadvantage of requiring additional space within the cavity for housing the stirrer assembly and results in a larger oven unit than is otherwise necessary.

To avoid this loss of available space, a proposal has been made which provides for mounting a conductive rectangular plate or paddle wheel for rotation about its axis within the waveguide that couples the microwave source to the cavity and confronts the microwave passage between the waveguide and the cavity. This proposal appears on pages and 146 in a book entitled, Microwave Heating" published in 1962 by the AVI Publishing Company.

That proposal, however, is not practical because an unduly large voltage standing wave ratio or VSWR, results when the paddle wheel is rotated to a position perpendicular to the sides of the Waveguide completely cutting off the transmission of microwave energy to the cavity. The microwave energy is then reflected back down the waveguide to the microwave source usually a magnetron. The reflected energy may cause the magnetron to mode or operate in an undesirable condition and usually results in the destruction of the magnetron. Moreover, since substantially the whole rectangular plate is rotated about its own axis, a portion of the plate on and about the axis remains transverse the width of the waveguide; hence, the change in phase of microwave energy propagated down the waveguide and the change in the reactive component or phase of the load seen by the microwave source at the input of the waveguide is limited. Desirably, the periodic change in phase should be as large as is permissible, while the change in the voltage standing wave ratio, seen by the microwave source should be minimal: This is desirable in order to effect a sufficient variation in frequency of a magnetron, and, thus, by-the introduction of different frequencies within the oven cavity, obtains a large variation in the number of different modes excited therein, while still protecting the source from reflected energy. Hence, because of the lim ited phase change, the ability of the paddle wheel to provide uniformity of heating within the cavity is limited.

Therefore, it is an object of the invention to provide a mode stirrer located outside the oven cavity and within the waveguide for maximizing the number of modes excited within a microwave oven.

It is another object of the invention to provide a mode stirrer for obtaining uniformity of heating, which does not require space within the oven cavity, permitting the manufacture of a more compact microwave oven.

It is a further object of the invention to feed microwave energy into the oven cavity at more than one location and to vary periodically the phase of microwave energy fed at one location relative to the other location.

It is a further object of the invention to provide an inthe-waveguide mode stirrer and assembly which avoids the creation of unduly large voltage standing wave ratios for protection of the microwave source and which produces maximum phase changes to ensure uniformity of heating.

It is another object of the invention to provide a mode stirrer and feed passages for a microwave oven that is mechanically rugged, simple, and inexpensive; and provides uniformity of heating within the oven cavity without theme of a fan-like apparatus.

In accordance with the present invention, a waveguide couples energy between a microwave source and the oven cavity. At least one microwave passage or iris is contained in a side of the waveguide abutting a cavity wall. A support member which is substantially transparent to microwave energy, such as an insulator, is mounted for rotation within the waveguide about an axis spaced between the one microwave passage and a short circuited or closed end of the waveguide. An elongated strip of conductive material for reflecting microwave energy is supported by the support member of insulator material spaced from the axis of rotation. A motor is coupled to the axis of the support member for rotating it with its supported conductive strip within the waveguide. Advantageously, in accordance with a specific embodiment of the present invention, a second waveguide passage or iris is contained in the side of the waveguide abutting the cavity wall, and this second passage is located between the support member and the closed end of the waveguide. In another em bodiment of the invention, the support member is conveniently of a cylindrical shape having a slot parallel to a cylindrical wall in which the elongated conductive strip is supported.

The foregoing and other objects and advantages of the invention will become apparent from a reading of the following detailed description taken in view of the drawings in which:

FIGURE 1 shows a pictorial view of the microwave oven embodying the invention; and

FIGURE 2 is an illustration of a support member for the elongated conductive strip utilized as an alternative to the support member illustrated in FIGURE 1.

FIGURE 1 shows a microwave oven unit having a cavity 1 bounded by conductive walls and including a door 2 through which objects to be heated may be placed into the cavity. A source 3 of microwave frequency energy, which is a conventional magnetron, is connected to the cavity by a waveguide 4. Waveguide 4 is shown with a section cut away in order to illustrate the internal elements more clearly. Included within waveguide 4 are a pair of spaced microwave passages or irises 6 and 7 through which the microwave energy from source 3 is coupled to the cavity. The passage 6 is known as a susceptibly stubbed iris and comprises two slots transverse the width of the waveguides: The second passage 7 is of the type known a a resonant iris. These passages are hereinafter described'in greater detail. A short circuited or closed end 8 terminates waveguide 4.

As is shown, these microwave passages, 6 and 7, may be opened as shown, or sealed with a material transparent to microwaves but impervious to vapors, in order to prevent vapors, such as food vapors, from entering the waveguide. The waveguide 4 additionally contains a support member 9 which is made from a material transparent to microwave energy. In one preferred embodiment, support member 9 has the geometry of a cylinder. An elongated conductive strip 11, which reflects microwave energy, is supported by member 9. The conductive strip 11 is covered with an electrical insulating material or potting compound, not illustrated, which has a high dielectric strength in order to prevent any possible arcing between conductive strip 11 and the conductive walls of the Waveguide. In the preferred embodiment, the support member 9 is constructed of a material commonly known by the trademark, Teflon; a product manufactured by the Du Pont Company. Other similar materials, such as nylon or polypropylene may be used instead.

Support member 9, and hence, the conductive strip 11, attached thereto, are mounted within the waveguide 4 about an axis of rotation located between the first iris 6 and the second iris 7 or closed end 8. In the preferred embodiment, as is shown, rotatable mounting of the member 9 is accomplished by a tapped hole within the member 9 and a coupling or shaft 14 having an outer screw portion 115 and a tapped hole 16. The waveguide 4 contains a hole, not illustrated, confronting the tapped hole 10 of the member 9. The shaft 14 is screwed into the hole 10 through the hole in the upper wall of the waveguide. The upper wall of the waveguide abuts the edge of the shaft 14 to act as a bearing surface. However, as is apparent, a washer may be utilized to act as a bearing surface if placed between the upper wall of the waveguide and edge of shaft 14. The shaft of a small electrical motor is coupled to shaft 14 to rotate support member 9. Since the member 9 and conductive strip 11 are relatively lightweight, the motor 12 is required to produce only a very small torque; hence, motor 12 may be any small-sized and relatively inexpensive fractional horsepower motor.

Suitable brackets, not illustrated, support motor 12 upon any convenient location in the oven unit, including the waveguide. Although FIGURE 1 illustrates a particular type of shaft means for rotatably mounting the support member 9 and conductive strip 11, it is apparent that other forms of shafts, such as a nonconductive shaft extending completely through the member 9 and connected to bearings located in the upper and lower walls of the waveguide 4, could be used instead.

Waveguide 4 is attached to the top wall of the cavity by a flange 5. This flange is sealed to the wall of the cavity in the conventional manner to prevent microwave energy from leaking to the exterior. Moreover, in the preferred embodiment, the upper wall of cavity 1 contains microwave passages identical to irises 6 and 7 underlaying the latter to permit the passage of microwave energy from the waveguide into the cavity. However, as is apparent, other forms of equivalent microwave passages may be used. For example, the upper wall of cavity 1 may contain a cut away portion corresponding to the shape of waveguide 4 and waveguide 4 containing the passages 6 and 7 is then located directly over or within this opening. Moreover, in accordance with the teachings of the present invention, spaced irises 6 and 7 may be of shapes or sizes other than that utilized in the preferred embodiment of FIGURE 1 and may consist of many forms of distributed passages. Also, the member 9 preferably supports conductive strip 11 about one-quarter guide wavelengths at the design frequency from the axis, but may be of any particular shape desired, since the only substantial requirement in shape is that it support the conductive strip 11 for rotation within the waveguide about an axis, other than an axis through the conductive strip 11, as found in the prior art paddle wheel.

The cavity 1 is conventionally constructed of stainless steel. Stainless steel, although it reflects microwave energy, is relatively lossy as compared to other metals and thus affords some load upon source 3 at all times even though no load or objects to be heated are within the oven cavity. This characteristic, thus, provides some degree of overload protection for magnetron 3.

The magnetron 3 is symbolically illustrated in FIGURE 1, and is connected in the conventional manner to a source of electrical energy which operates the magnetron in the usual manner. Likewise, the motor 12 is connected to a source of AC. Details of the foregoing including the usual on-off switch, circuit breakers, and details of the power supply have been omitted; since they are well known, are connected in a conventional manner, and do not add to the present invention.

Upon energization, source 3 produces microwave energy which propagates down waveguide 4 and enters the cavity 1 through irises 6 and 7. Motor 12 rotates the support member 9 and moves elongated conductive strip 11 in a circle within waveguide 4. As is apparent, conductive strip 11 rotates to positions substantially parallel to the side walls of the waveguide, and the microwave energy from source 3 passes into cavity 1 through both irises 6 and 7. Additionally, conductive strip 11 rotates to positions substantially parallel to the closed end 8 of the waveguide 4 and substantially blocks the microwave energy from iris 7. These latter positions occur at two dilferent locations; where strip 11 is proximate the iris 7 and where strip 11 is more proximate the iris '6. The space between these two locations is a full diameter of member 9. Preferably, this distance is approximately a half guide wavelength at the design frequency. Thus, as strip 11 rotates in the manner described, the impedance of the waveguide changes and varying amounts of microwave power are directed through each of irises 6 and 7 during the course of this rotation.

A magnetron has several inherent characteristics which are of significance to the present invention. As is known, the operating frequency of the magnetron is dependent upon the impedance of the load, which in the present instance is the load seen by the magnetron at the entrance to waveguide 4. In any particular magnetron, these characteristics are graphically displayed by the well-known Rieke diagram. Characteristcs, such as power output and frequency of oscillation of a magnetron are plotted upon a Rieke diagram as a function of the magnitude and the phase of the voltage standing wave ratio (VSWR).

In all magnetrons there is a region of possible operation in which the magnitude of the voltage standing wave ratio and the phase of the voltage standing wave ratio cause the magnetron to mode or operate at an undesired frequency, and which if continuously operated in this region causes the destruction of the magnetron. This region is also outlined on the Rieke diagram. Thus, precautions are taken to prevent any change in load seen by the magnetron from causing the magnitude and the phase of the voltage standing wave ratio from appearing within this undesirable range. In the present instance, the location of a microwave passage, such as iris 6, closer to the source end of waveguide 4 than the reflecting conductive strip 11, prevents the voltage standing wave ratio from becoming unduly large. Regardless of the location of the conductive strip 11, it does not block microwave energy from iris 6. This is in marked contrast to the paddle wheel devices of the prior art which are located between the source and the microwave passage. At certain angular positions, such paddle wheels completely reflect all energy propagated down the waveguide back to the source, creating an unduly large voltage standing wave ratio. Such a large voltage standing wave ratio (VSWR) when coincident with an unforeseen phase shift, causes the magnetron to operate in the forbidden region.

The second effect apparent as elongated strip 11 rotates about the axis of shaft 14 between the iris 6 and the closed end 8 of waveguide 4 is the change of frequency of magnetron 3 caused by the change of impedance of the load seen by magnetron 3. This phenomenon, commonly known as frequency pulling is also demonstrable with a Rieke diagram. The term pulling has a very technical meaningthe change in frequency caused -by a change in VSWR phase of the voltage standing wave ratio, defined as the distance of the node or minimum of voltage, expressed in radians or wavelengths, from the magnetron input; while maintaining the voltage standing wave ratio at a magnitude of 1.5. However, it is herein used in a broader sense; that is, the frequency change produced by a VSWR change or a VSWR phase change, or both.

In the present invention, it has been found that the change in voltage standing wave ratio caused by the presence of a reflective body within the waveguide as part of the load seen by the magnetron, such as strip 11, is dependent more upon the height of such strips and that the VSWR phase change of the voltage standing wave ratio, again as part of the load seen by magnetron 3, is dependent more upon the length of the conductive strip transverse the width of the waveguide, such as the portion of the length of strip 11 rotated transverse the width of the waveguide. In the preferred embodiment it is desirable that the length of the strip 11 amount to about one-third the circumference of the support member carrying the conductive strip or, in other words, about the width of the waveguide. Thus, substantially parallel, that is as parallel as a curved strip can be, to closed end 8 of the waveguide where it produces a maximum phase shift, and then to a position substantially parallel to the side walls of waveguide 4 where a minimum phase shift is attained. This rotation of strip 11 causes a very large change in VSWR phase of the voltage standing wave ratio seen by the magnetron between the minimum-maximum values in the phase of the voltage standing wave ratio.

With a conductive strip of length greater than onethird the circumference of the support cylinder, or greater than the width of the waveguide, it is apparent that some portion thereof always protrudes transverse the width of Waveguide 4; and hence, maintains some value of VSWR phase or reactive value larger than a minimum value otherwise possible. Hence, having a larger minimum value of VSWR phase results in a smaller phase change, the difference between the maximum and minimum VSWR phase, seen" by the magnetron during rotation of strip 11. The same reduction results if the elongated strip is substantially shorter in length than the width of the waveguide, however, in such instances, the maximum VSWR phase shift is reduced.

Further, in the preferred embodiment, the edges of conductive strip 11 are chamfered, which effectively changes the height of conductive strip 11 at those portions. Hence, as strip 11 rotates from a position substantially parallel to the side walls of waveguide 4 transverse the width of waveguide 4 progressively to the blocking position parallel to closed end 8, the voltage standing wave ratio, because of its dependency upon the height of the conductive strip varies in magnitude during the period in which conductive strip 11 is first rotated across the width of the waveguide.

The frequency of the magnetron 3 is thus pulled" through a large number of frequencies as reflecting strip 11 rotationally changes position. Having different frequencies propagating down waveguide 4 through the irises 6 and 7 and into cavity 1 caused either by this pulling or by changes in magnetron anode current produces large variations in the types of modes temporarily created and maintained within cavity 1. This prevents the creation of a standing wave field distribution of a single mode and characteristic of a single frequency conductive to the creation of hot and cold spots, since this modal pattern changes as the conductive strip 11 is rotated through its various positions.

An effect occurring during the rotation of conductive strip 11, in addition to the VSWR and VSWR phase change, is the change in phase of the instantaneous microwave energy produced between the first iris 6 and the second iris 7. Because the reflective strip 11 acts as a variable impedance connected between two locations along a transmission line, herein Waveguide 4, the change in impedance which it produces changes the phase of the instantaneous microwave field at one location relative to another location along the waveguide in the conventional manner. Hence, when the combined wave front of microwave energy, emanating from the irises 6 and 7, is considered, it is apparent that variations in phase between the two irises produce a maximum field within the cavity which varies in location or position along the cavity wall in a manner similar to the phasing of a plurality of radio frequency antennas. As is apparent, the mode sustained by the cavity is one which has its maximum or antinode at the same location of the maximum field coupled thereto. Thus, as the point of field maximum is shifted, different modes are coupled and momentarily sustained by the cavity.

Although the theory of operation of the invention is at present uncertain and unusually complex, the foregoing explanation of the operation is olfered as the best available based upon known phenomena in order to ofier some understanding of the unusually complex relationships that appear to control the success of the invention and which normally require an empirical design. The uniformity of heating produced by the present invention is consistently superior to that obtained with devices such as the paddle wheel, heretofore available.

FIGURE 2 shows another embodiment of the invention in which the rotatable conductive strip is located within the waveguide 4 in the same manner as the corresponding elements illustrated in FIGURE 1. In this embodiment a slot 18 is cut into' the support member 9' along a cylindrical surface at a predetermined radial distance from the tapped hole 10', less than the radius of the cylindrical portion of the insulator body. This slot 18 is long of such length and depth that it can contain the elongated conductive strip 11. The conductive strip and the remaining portion of the slot 18 are covered with a suitable potting material 19 having a high dielectric strength. With this arrangement, the conductive strip 11 is more permanently and rigidly supported by the support member 9; and both the member 9 and the potting material 19 act to prevent any electrical arcing between conductive strip 11' and the walls of waveguide 4. The shape of the member 9' is formed by the intersection of a cylinder and two parallel planes each of which are spaced from the axis of the cylinder by a distance smaller than the radius of the cylinder. The member 9' thus has two cylindrical sides and two fiat sides, 17 and 20. The purpose of this shape is to simplify one procedure of manufacturing the invention.

In one such manufacturing procedure, the waveguide 4 including the irises 6 and 7 shown in FIGURE 1, is first formed. In order to insert the support member 9 containing the supported conductive strip 11' into the waveguide, the diameter of the cylinder is made slightly smaller than the longer dimension of the rectangle forming the passage 7, while the distance between the two parallel flat sides 17 and 20 is made slightly smaller than the small dimension of the rectangular passage 7. The member 9 is inserted through the iris 7 into the waveguide 4 by hand and manipulated so that its tapped hole 10 appears at the hole, not illustrated, in the upper wall of the waveguide. The shaft 4 may then be screwed into the tapped hole 10 to hold the member 9' in place until other steps in the manufacturing process can be completed. Although this step of the manufacturing procedure has been described, this description is not intended to limit the invention or the method by which the invention is manufactured.

Referring again to the preferred embodiment of FIG- URE 1, the microwave passage means utilized, as previously mentioned, is a form of distributed feed which contains a first passage or iris known as a susceptibly stubbed iris formed by the two narrow slots 6 transverse the width of the waveguide, and a second passage or iris known as a resonant iris formed by a rectangular opening 7 the center of which is located approximately one-half wavelength at the design frequency from the effective midpoint of the susceptibly stubbed iris 6 and approximately one-fourth wavelength at the design frequency from the short circuited or closed end portion 8 of waveguide 4. The shorter side of the rectangular opening runs across the width of waveguide 4. The purpose of this type of distributed feed is twofoldfirst, for introducing equal amounts of power at each of two respective locations along the top wall of the cavity when the conducting strip is rotated to a position parallel to the longitudinal walls of the waveguide 4, and second, for maximizing the number of modes produced within the cavity. Each of these functions enhances the uniformity of heating of objects placed within cavity 1. The location of the short circuited waveguide end 8 enhances the introduction of microwave energy through opening 7, because a substantially high impedance is seen by microwave energy attempting to pass this opening and progress further down the waveguide. Essentially this is equivalent to a quarter wavelentgh short circuited stub, well known in transmission line theory. In the preferred embodiment, it is desired to have resonant iris 7 located adjacent the closed end 8 of the waveguide, rather than susceptible stubbed iris 6.

The narrow slots 6, as is known, favor the passage of electric field or E waves, while the resonant iris 7 favors the passage from the waveguide into the cavity of magnetic field or H waves, although some amounts of the other field are also admitted, especially by iris 7. The waveguide 4, coupling the microwave energy from the source 3 to the cavity 1, is designed to support the propagation therebetween of what is known as a transverse electric wave; that is, a propagating wave containing both electric fields E and magnetic fields H in which.

the magnetic field H has a vector parallel to the direction of propagation, and the electric field E has its vectors entirely transverse to the direction of such propagation. In particular, the waveguide is designed to support the propagation of a TEOl wave. Many of the standard texts or waveguides relate the mathematical analysis of such phenomena. Moreover, an oven cavity, such as 1, is capable of sustaining different modes of field distribution depending upon whether an E field or an H field is coupled from the waveguide into the cavity. Therefore, by introducing an E field at one location and an H field at another, a greater number of modes are excited and the desired result of maximizing the number of excited modes for obtaining uniformity of heating is obtained where it is generally not otherwise possible with two couplings both of which couple either E fields or H fields.

Further, as is known, a high frequency passage, such as the disclosed irises, possesses a value of radiation resistance, determined in part by the dimensions of the passage and the frequency of the microwave energy. Since it is desired to distribute the microwave power evenly throughout the cavity, it is desirable to have equal amounts of power enter the cavity through each of the two irises 6 and 7. This would not be possible if the two irises possessed equal values of radiation resistance, because in that instance, maximum power would be coupled through the first iris and less through the second iris. Hence, in the preferred embodiment utilizing the distributed feed, the irises are dimensioned so that at the design frequency they are of unequal radiation resistance.

Although the preferred embodiment of the invention utilizes a distributed feed, other modification-s embodying the invention are also possible; for example, it is possible to utilize a single iris, such as a resonant iris like passage 7, for the microwave passage means. It preferably is located in the bottom wall of the waveguide, substantially in the center of the top wall of the cavity 1. In this instance, the support member is placed in a location closer to end 8 within the waveguide so that it remains between the iris and the short circuited end 8 of the waveguide.

Of course, it is to be understood that this invention is not restricted to the particular details as described above, as many equivalents will suggest themselves to those skilled in the art. The foregoing embodiments, it is understood, are presented solely for purposes of illustration and are not intended to limit the invention as defined by the breadth and scope of the appended claims.

What is claimed is:

1. A microwave oven comprising: a source of microwave energy; a cavity for receiving objects to be heated; a waveguide connected between said source and said cavity for coupling microwave energy between said source and said cavity; said waveguide including a short circuited end portion remote from said source, a first microwave passage means spaced from said short circuited end for feeding microwave energy from the waveguide into the cavity; a support member contained within said waveguide rotatably mounted about an axis of rotation between said first microwave passage means and said short circuited end; said support member being of a material substantially transparent to microwave energy; an elongated conductive strip supported by said support member spaced from and substantially non-coplanar with said axis of rotation for rotation with said support member about said axis of rotation for reflecting variable amounts of micro wave energy during rotation; and means for rotating said support member about said axis of rotation so that said reflective strip is moved in a narrow path located about said axis between said passage and said short circuited waveguide end that traverses substantially a major portion of the width of said waveguide at two spaced positions along the length of said waveguide.

2. The invention as defined in claim 1, further comprising a second microwave passage means within said waveguide spaced from said first microwave passage means and located between said axis of rotation and said short circuited waveguide end for feeding microwave energy from the waveguide into the cavity from a second location.

3. The invention as defined in claim 2, wherein said first microwave passage means comprises a pair of narrow adjacent slots transverse the width of said waveguide and said second microwave passage means comprises a rectangular opening larger in area that said slots.

4. The invention as defined in claim 3, wherein said support member is of a substantially cylindrical geometry and said axis of rotation substantially coincides with the axis of said support member.

5. The invention as defined in claim 4, wherein said support member contains a 'slot opening within a base portion; and wherein said elongated conductive strip is supported within said slot; and further comprising a covering material of high dielectric strength covering said slot.

6. A microwave oven comprising: a source of microwave energy; a cavity for receiving objects to be heated; a waveguide connected between said source and said cavity for coupling microwave energy therebetween; said waveguide including a plurality of microwave passages for feeding microwave energy between said waveguide and said cavity at different locations; a microwave energy reflecting surface within said waveguide between two of said plurality of microwave passages for changing the phase of microwave energy between said microwave passages in response to a change in position; and means connected to said reflecting surface and having an axis of rotation spaced from said substantially non-coplanar with said reflecting surface for moving said reflecting surface in a narrow path about said axis of rotation that includes traversing substantially the width of said waveguide at two spaced positions along the length of said waveguide substantially in between said microwave passages.

7. A microwave oven comprising: a magnetron for generating microwave energy; a cavity for receiving objects to be heated; waveguide means connected between said magnetron and said cavity for coupling microwave energy therebetween and presenting a VSWR at a VSWR phase to said magnetron; said waveguide including a short circuited end and a first passage means spaced from said short circuited end for feeding microwave energy between said waveguide and said cavity; microwave energy reflecting surface within said waveguide located between said passage means and said short circuited end; an axis of rotation spaced from said reflecting surface; said reflecting surface being spaced from and substantially noncoplanar with said axis; and means for rotating said reflecting surface about said axis of rotation so that said reflecting surface is moved in a narrow surface path which traverses substantially a major portion of the width of said waveguide at two spaced positions along the length thereof for varying the VSWR phase presented to said magnetron.

8; The invention as defined in claim 7, further comprising: a second passage means within said waveguide for feeding microwave energy having a positional phase from said waveguide into said cavity; said second passage means spaced between said microwave energy reflecting surface and said short circuited end of said waveguide; and wherein said microwave energy reflecting surface additionally varies the instantaneous phase of microwave energy supplied at said second passage means relative to the instantaneous phase of microwave energy at said first passage means.

9. The invention as defined in claim 8 wherein said first microwave passage means comprises a pair of adjacent narrow transverse slots and wherein said resonant iris comprises an opening larger in area than said slots.

10. The invention as defined in claim 4 wherein the length of said reflective strip is slightly larger than the width of said waveguide; and wherein said reflective strip is curved in the arc of a circle.

11. The invention as defined in claim 6 wherein said reflecting surface comprises an elongated strip of conductive material.

12. The invention as defined in claim 11 wherein said elongated strip has a length slightly larger than the width of said waveguide; and wherein said strip is curved in the arc of a circle.

References Cited UNITED STATES PATENTS 2,909,635 10/1959 Haagensen 219-1055 RICHARD M. WOOD, Primary Examiner.

L. H. BENDER, Assistant Examiner,

P0405 UNITED STATES PATENT OFFICE 9 CERTIFICATE OF CORRECTION Patent No. 3, L 39, Hg P Dated rpm 15; 1969 Inventor(s) R. L. Cougoule It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

r In Column 9, Line L O before the word substantially delete "said" I and substitute therefor--and-.

SIGNED AND SEALED SEAR M flemhenl wxmmm E. saHuYLER, m- Ea M Commissioner of Patents Amsting Officer 

